Case 826 -- A 31 Year Old Female with a Destructive Clival Mass

A 31 year old woman presented with worsening neck pain for 2 weeks. She was treated for muscle spasms at an urgent care clinic with muscle relaxants and narcotic pain medication, but had no relief of symptoms. On physical exam, she had focal tenderness in the midline at the base of the occiput and resistance to any motion of the neck. Routine laboratory examination was unremarkable. There was no associated history of trauma. CT scan showed a large, expansile, and destructive mass in the right clivus extending into the right petrous bone with associated right medulla effacement (Fig 1). Follow-up with MRI again showed a 5.0 x 3.0 x 3.0 cm expansile, destructive mass in the right clivus and along the right anterolateral margin of the foramen magnum/base of the skull (Fig 2).

MICROSCOPIC PATHOLOGY

Microscopic examination showed large areas of calcification separated by fibrous material (Fig 3). On higher power, the calcification appears chunky and plate-like (Fig 4). The fibrous tissue is composed of spindle cells without mitoses, necrosis, or pleomorphism. Some areas have an epithelioid appearance (Fig 5). Scattered throughout the lesion are multinucleated giant cells (Fig 6). S100, EMA, GFAP, Desmin, and pan keratin were negative. Reactive stromal myofibroblasts were seen with SMA, and CD163 highlighted histiocytes. What is the diagnosis?